Coating for cathodically protected structures

ABSTRACT

A CATHODICALLY PROTECTED METAL ARTICLE, SUCH AS A PIPELINE, TANK, OR VESSEL, IS COVERED WITH A FIRST COATING OF CHLORINATED RUBBER, AND A SECOND, OUTER COATING OF A BITUMINOUS MATERIAL WHEREIN EITHER OR BOTH COATINGS CONTAIN A DISPERSION OF NOT LESS THAN 5% BY WEIGHT OF CALCIUM CARBONATE. THE THUS COATED ARTICLE EXHIBITS IMPROVED CORROSION RESISTANCE.

United States Patent O 3,707,450 COATING FOR CATHODICALLY PROTECTEDSTRUCTURES Herbert E. Townsend, Hellertown, Pa., assignor to BethlehemSteel Corporation No Drawing. Filed Aug. 5, 1970, Ser. No. 61,475 Int.Cl. C23f 13/00 US. Cl. 204147 7 Claims ABSTRACT OF THE DISCLOSURE Acathodically protected metal article, such as a pipeline, tank, orvessel, is covered with a first coating of chlorinated rubber, and asecond, outer coating of a bituminous material wherein either or bothcoatings contain a dispersion of not less than by weight of calciumcarbonate. .The thus coated article exhibits improved corrosionresistance.

BACKGROUND OF THE INVENTION This invention relates to an improvement incorrosion resistant coatings for metal structures and particularly forcathodically protected structures.

Underground pipelines, such as those used for transcontinentaltransmission of gas, are protected throughout much of their length bycathodic protection. This protection may be effected by means of animpressed current or by use of sacrificial anodes.

Gas transmission lines are generally subjected to high stress (up to 72%of the nominal yield strength) owing to high pressure. This fact,coupled with corrosive, low resistivity (less than 10,000ohm-centimeters) soils encountered in the course of long pipelines,places heavy demands on any cathodic protection system.

Cathodic protection is effective for pipelines without the use ofexternal coating. However, the general practice is to coat the pipe witha relatively heavy coating of a bituminous material, usually with a thinprimer coating adjacent to the metal substrate, in order to minimize theamount of current required for a given amount of protection.

While underground structures, such as pipelines and storage tanks, areusually freeof corrosion when a bituminous coating applied theretoremains intact, disbonding of the coating, i.e. the tendency of thecoating to become separated from the metal substrate, is a constantproblem, particularly in an area immediately adjacent to a coatingdefect. Disbonding increases the area of exposed metal surface. Thismeans that, for a fixed amount of impressed cathodic current, thecurrent density (current per unit area) decreases. Hence, the degree ofcathodic protection, which is determined by the polarization of themetal-soil interface measured against a standard reference electrode,will also decrease. Alternatively, if the current is adjusted in orderto maintain the same current density and degree of cathodic protection,greater amounts of current are required as the coating proceeds todisbond.

In some cases, organic compounds such as imidazolines, ethanolamines,sulfonic acids, etc., have been added to the coating in an attempt toretard disbonding.

Another approach to increasing the effectiveness of cathodic protectionfor underground structures has been to add calcium carbonate to the soilsurrounding the structure in order to promote the deposition of aprotective, calcareous deposit.

It .is an object of this invention to decrease the amount of disbondingbetween an underground cathodically protected metal structure and itsprotective coating.

It is another object to minimize the effects of any dis- 3,707,450Patented Dec. 26, 1972 bonding which may occur in the coating of suchstructures.

It is a further object to reduce the amount of current required in acathodic protection system for coated underground metal structures.

SUMMARY OF THE INVENTION I have found that underground metal pipelinesand underground metal storage vessels, which are cathodically protected,can be coated with a material containing a calcareous inhibitor, wherebydisbonding can be decreased substantially or eliminated, and the currentnecessary to effect cathodic protection to the metal over long periodsof time can be reduced effectively as well.

Briefly, this invention comprises coating the surface of, for example, ametal pipeline with a relatively thin primer coating of chlorinatedelastomer, such as rubber or synthetic rubber. The primer is coated witha bituminous material of the type preferably represented by coal tar.The outer, bituminous coating is impregnated with at least 5% calciumcarbonate, while the primer, preferably, also contains about 5% or moreof calcium carbonate.

In cathodically protected pipe bearing the coating of this invention,not only is the amount of disbonding reduced, but also, when disbondingdoes occur, corrosion protection can be maintained with a substantialreduction in protective current, as compared to prior art protectivemeasures.

DETAILED DESCRIPTION Is one example by which my invention can beperformed, lengths of steel pipe, of the type used in transmission ofnatural gas, are cleaned by shotblasting and then coated with achlorinated rubber primer containing about 25% by weight of limestone(less than IOU-mesh, U.S. Std. Sieve Series). The primer coating isprepared by mixing calcium carbonate, in the form of limestone, to acommercial grade of chlorinated rubber primer solution, thus forming asuspension. Commercial chlorinated rubber primers, intended asprime-coats for coal tar enamel, are available in liquid form with thechlorinated rubber dissolved in an appropriate solvent such as xylene.The suspension, which contains from 50% to 55% solvent by weight, isapplied to the outer surface of the pipe at a rate of one gallon ofsuspension for every 650 to 850 sq. ft. of pipe surface, resulting in aprimer coating thickness, after evaporation of the solvent, of about0.001 inch. Once the primer is dry and firmly set on the pipe surface,the pipe is ready for application of the bitumunous outer coating.

In preparing the outer coating, coal tar enamel-a mixture of topped coaltar, inert filler and, optionally, a plasticizeris heated to a freelyflowing condition, and ground limestone (less than -mesh is incorporatedwith the hot tar in an amount equal to about 25% by weight. After thelimestone has been thoroughly mixed with the molten coating the mixtureis ready for application as the outer coating to the pipe. The hot tarenamellimestone mixture, at a temperature of about 450 F., is pouredfrom a tank, downwardly onto a horizontal section of pipe which isrotating about its longitudinal axis, whereby the pipe is progressivelycompletely covered with the mixture to a thickness of about 0.1 inch.

In general, control of the thickness of the coating is maintained byapplying an overlap of felt, fiberglass or kraft paper, to squeeze thecoating to the desired thickness. This overlap also gives protection tothe coating during handling and installation of the pipe.

Coating materials of the type used for coating individual lengths ofpipe are also used for coating any bare areas on the assembled pipeline,such as those areas existing where individual lengths of pipe have beenjoined by welding.

After the pipeline has been fabricated from coated sections and locatedin its pipeline trench, electrical connections are made for cathodicprotection by impressed current in the manner well known in the art.

The anodes used in the impressed current protection system may be anyconductive material such as copper, carbon, iron, etc. which willdeteriorate slowly and provide long service. The source of impresseddirect current may be, for example, a rectifier, generator or battery.In order to protect any pipe surface areas which may become exposedduring use, a cathodic voltage of at least -0.85 volt with respect to acopper-copper sulfate reference electrode should be maintained at themetal surface.

A pipeline prepared in the manner of this invention will require lesscurrent to protect the pipe surface than is required of prior artcathodic protection methods. The reason for the reduction in impressedcurrent is due to a decease in disbonding, and to the inhibitive actionof the coating of the invention in promoting the formation of protectivecalcareous films on any exposed metal surfaces and in pores in thecoating.

Reduction in current consumption by the practice of my invention resultsnot only in lower operating cost than in prior cathodic protectioncircuits, but also reduces the amount of cathodically evolved hydrogen.Hydrogen can be a source of embrittlement if absorbed by pipe or tanksconstructed of steels having a yield strength greater than 100,000p.s.i.

This invention has particular application to gas transmission pipelines,as these lines are usually maintained under an internal pressure, andany corrosion which might Weaken the walls of the pipe to the pointwhere rupture, and consequent explosion, could occur cannot betolerated. Likewise, the invention is quite applicable to undergroundtanks, particularly those under pressure.

Various alternatives will present themselves by which satisfactorycorrosion resistance can be obtained by application of this invention tounderground structures.

For example, the coal tar enamel used for the outer coating may beplasticized, semi-plasticized or unplasticized, depending on thetemperature conditions to which the structure is exposed. Plasticizingof the enamel is usually effected with ground coal. Bitumens other thancoal tar are operative, for example, petroleum asphalt or naturalasphalt can be used, although coal tar, in the form of coal tar enamel,has been found to be the most satisfactory.

Calcium carbonate in almost any form is satisfactory as the inhibitor inthe coating, as long as the carbonate is of a degree of finenessadaptable for thorough and uniform mixing with the primer or outercoating. In this regard, ground limestone is quite suitable if of afineness less than about lOO-mesh. The calcium carbonate may be added inthe form of ground dolomite. Other alkaline earth metal carbonates,including barium and strontium, may be used alone or jointly. Any ofthese alternative inhibitor materials should be of a size less than100-mesh in order to realize the greatest advantage from their use. Thebenefits of the invention may be obtained but to a lesser degree, withsomewhat larger particle size inhibitor.

The maximum amount of calcium carbonate which can be used in primer orouter coating is that amount which permits the primer material or thebitumen to flow freely at the recommended application temperature. Thus,the upper limit of calcium carbonate which can be used effectively inthis invention will vary, depending on the exact nature of the coatingmaterial.

Manufacturers recommended application temperature for unplasticized coaltar enamel ranges, generally, between 375 and 475 F., and forplasticized coal tar enamel, the application temperature is between 450and 550 F.

In tests conducted on one commercialtype coal tar enamel, to establishpermissible amounts of calcium carbonate, it was found that 42 weightpercent of calcium carbonate could be added to the fully plasticizedgrade at 450 F., while 54 weight percent calcium carbonate could beadded to the unplasticized grade at 428 F., without noticeably impedingthe fluidity of the mixture.

Calcium carbonate can be added to the primer in an amount equal to orgreater than those shown for the coal tar enamel, for in the case of theprimer, solvent is added to produce the proper fluidity.

In the matter of a primer, materials other than chlorinated rubber havebeen tried with indifferent success.

A series of test panels for the comparative study of the behavior ofinhibited and uninhibited coatings under simulated cathodic protectionwere prepared from 4-inch by 4-inch by fli-inch hot rolled carbon steelplates, sandblasted and cleaned in trichlorethylene vapor. Primer(chlorinated rubber) was applied by brush, after mixing in additions ofinhibitor for those test specimens requiring it. All primed and unprimedtest panels were coated by dipping them into a container of moltencoating bitumen (plasticized coal tar enamel) for a length of time(usually about 3 seconds) sufficient to result in a coating thickness of0.09:0.03 inch when removed and allowed to drain in air. Mixtures ofcoating bitumen and inhibitor were prepared by heating the bitumen tothe application temperature of from 450 to 490 F. and adding the'desired amount of inhibitor (25% by weight calcium carbonate for bothprimer and coating).

Preliminary viscosity tests were performed todetermine the maximumamounts of the various types of inhibitors which could be added to thecoating at the recommended application temperature. Amounts less thanthe maximum quantities thus determined were employed in subsequentdisbonding tests.

After coating the panels, the procedure for the disbonding testscomprised the following steps:

( l) A A-inch diameter hole (holiday) was drilled in the center of apanel through the coating to the bare steel to provide an intentionalholiday.

(2) A 3-inch diameter by 4-inch long glass tube, open at both ends, wascemented to the panel with epoxy cement in a manner to form a containerabout the holiday.

(3) A magnesium anode (1 inch by 1 inch by 4 inches) was electricallyconnected through a l-ohm resistor to a steel panel, and suspended in anelectrolyte, about 1 inch above the holiday. The anode was immersed inthe electrolyte for a distance of about 2 inches to provide cathodicprotection by means of the sacrificial magnesium anode. I p

(4') The electrolyte comprised 1% sodium chloride (NaCl), 1% sodiumsulfate(Na- S0 and 1% sodium carbonate (Na CO in 300 ml.'of distilledwater.

(5) At one week intervals, in a continuous -day test period, the liquidlevel was maintained by additions of distilled water, and the currentmeasured by determining the voltage across a one-ohm resistor with anelectrometer. I I,

(6) Following the 120-day exposure, the cell was dismantled, the bulk ofthe primer and outer coating was pried away from the holiday region witha spatula, and the disbonding measured. i

Linear disbonding measurements in centimeters were made on each testspecimen from which the coating had been removed after test; Thesemeasurements represent total disbonding, and were made from theperiph'ery of the holiday to the periphery of the opening made bythedisbonded coating. Primer-outer coating and primer-steel disbondingrepresent, respectively, the portion of the specimen where the outercoating has disbondedfrom the primer, and primer has disbonded from thebasemetal, and the sum of the two types of disbonding is referred to astotal disbonding in Table I below.

Table I lists results for two eries of tests, the first series having noinhibitor and the second series containing inhibitor in both the primerand outercoating. Three specimens were tested for each series. Currentis shown as an average of the weekly determinations.

nated rubber primer containing 25% CaCOz powder.

1 No inhibitor. 3 Inhibitor.

The average reduction in current required for the inhibited specimenswas 52%, while the average reduction results for a number of thedifferent variables considered for this second set of tests. As in TableI tests, specimens were tested in triplicate and the current resultsaveraged. While the differences in current and disbonding betweeninhibited and uninhibited specimens is not as great in Table II as inTable I, it will be appreciated that the 30- day tests are relativelyshort term as compared with those of four months.

Applicants invention is designed to give improved protection topipelines and underground tanks for a matter of years, and it is oversuch long-range use that the greatest bene-fits from the invention arerealized. The tests used to supply results for both tables areaccelerated tests with.the results of Table I showing the increasinglygreater benefits obtained with longer use of the inhibited coatings.

The purpose of the test results in Table II, as previously mentioned, isto provide a means of comparing variables, there being sufiicientshowing of improvement in corrosion resistance shown by the inhibitedspecimens, even in the relatively short -day tests, to point up certainpreferred practices in performing my invention.

TABLE II Average Total dis- Primer Coating total onding current, centi-Test series No. Type Inhibitor Type Inhibitor milliamps meters,

I specimen:

1 1, 610 0. 83 2 Chlorinated rubber- None Plasticized coal tar. None 1,980 0. 60 3 1,790 0. 80

1, 540 0. 47 do 25% CaCOareagent ..do 25% CaCO; reagent.-. 1,260 0.47 31, 370 0. 50 III specimen:

1 1,360 0. 50 do 20% CeCOa reagent ..do 20% CaCO: reagent 1,500 0.451,290 0.45

1,340 0.52 .do 25% limestone passed 400 mesh ..do 25% limestone passed400 mesh. 1, 470 0. 52 3 1,480 0.55 V specimen:

1 1,530 0. 62 do 25% dolomite passed 400 mesh do 25% dolomite passed 400mesh.. 1,330 0. 1, 350 0.55

1, 560 0. 67 do None d0 25% 021005 reagent 1,010 0.67 1, 270 0. 70

1,100 0.42 do 40% limestone passed 400 mesl1 d0 35% limestone passed 400mesh. 1, 190 0. 48 1,260 0. 48

A 1,980 0. 42 do 25% Ca'COs reagent do None 1,750 0.57 2, 020 0. 20

l Koppers 708 external tar enamel.

in total linear disbonding for the same specimens was 67%.

The results in the above table show outstanding differences, in bothcurrent required and amount of disbonding, between the specimensinhibited by the method of this invention, Test Series II, anduninhibited specimens, Test Series I.

Another set of tests was run for a period of 30 days. In these tests,specimens were prepared and tested in exactly the same manner as inTable I tests, except for the shorter test period. By virtue of thisshorter testing period, the inventor was enabled to test hundreds ofspecimens in a reasonably short time.

From the table, it will be noted that when both primer and outer coatingwere inhibited with at least 20% of a calcium carbonate or dolomiticmaterial, the total disbonding was measurably decreased over theresultant total disbonding when no inhibitor was used. Inhibitor inamounts as low as 5% produce beneficial results, a lthough to a lesserdegree than for amounts of 20% inhibitor or greater. When no inhibitorwas used in the primer, but was included in the outer coating as in TestSeries No. VI, total disbonding was not appreciably reduced. However,the average total current required for protection in the case of TestSeries VI was an improvement over that required for Test Series I inwhich no Table II, given below, is a representative listing of inhibitorwas used in the primer or in the outer coating.

7 Similar improvement in total current required was noted for the TestSeries from 11 to V and VII. In Series No. VIII, with inhibitor in theprimer only, there was improvement in total disbonding, but no reductionin current required over Series I with no inhibitor.

In all specimens for which results are shown in Tables I and II, theprimer thickness was within the range of from 0.0005 to 0.0015 inch, andthe coating thickness was within the range of from 0.065 to 0.120 inch.

All percentages given above and in the appended claims represent weightpercent.

I claim:

1. A process for protection of the outer surface of a metallic articleexposed to a corrosive underground environment which comprises applyingto said surface an elastomer primer coating and an outer coating of abitumen wherein each of the coatings contains more than by weight of analkaline earth metal carbonate, and subjecting the thus-coated articleas a cathode to an electric current sufficient to retard corrosion ofthe surface of said metallic article.

2. A process according to claim 1 wherein the metallic article is aferrous article, the elastomer is chlorinated rubber and the bitumen iscoal tar enamel.

3. A process according to claim 2 wherein the ferrous article is a pipeor tank.

4. A process according to claim 2 wherein the primer coating and outercoating each contains not less than 20% by weight of calcium carbonate.

5. A cathodically protected metal pipeline having the outer surface ofthe pipeline protected by a coating comprising: I

(a) a first layer of chlorinated rubber bonded to the outer metalsurface of's aid pipeline,

(-b) a second layer of bitumen bonded to said first layer wherein eachof said layers contains a dispersion of more than 5% by weight ofcalcium carbonate, and

(c) an anode in electrical circuit with said pipeline as the cathode.

I 6. An article according to claim' 5 wherein said first and secondlayers each contain at least 20% by weight of calcium carbonate.

7. An article according to claim 6 wherein the first layer has a coatingthickness between 0.0005 and 0.0015 inch and the second layer has acoating thickness be-' tween 0.065 and 0.120 inch.

References Cited UNITED STATES PATENTS 2,054,769 9/1936 Holtz 138-1452,365,427 12/ 1944 Moore 138145 3,001,919 9/1961 Petrocokino 204-1483,354,063 11/1967 Shutt 204-197 3,409,525 11/1968 Taylor et al. 204-1473,484,349 12/ 1969 Vrable 204-496 3,553,094 1/1971 Scott et a1 204197TA-HSUNG TUNG, Primary Examiner US. Cl. X.R.

-117'79, 92, 128.7;"133, 138-446, 178, Dig. 6;

